The present invention relates to a process for the gas-phase catalytic polymerisation, particularly for the polymerisation of xcex1-olefins, carried out in two or more interconnected polymerization zones to which one or more monomers are fed in the presence of a catalyst under polymerization conditions and from which the produced polymer is discharged. The development of olefin polymerization catalysts with high activity and selectivity, particularly of the Ziegler-Natta type and, more recently, of the metallocene type, has led to the widespread use on an industrial scale of processes in which the polymerization of olefins is carried out in a gaseous medium in the presence of a solid catalyst. A widely used technology for gas-phase polymerization processes is the fluidized-bed technology. In fluidized-bed gas-phase processes, the polymer is confined in a vertical cylindrical zone. The reaction gases exiting the reactor are taken up by a compressor, cooled and sent back, together with make-up monomers and appropriate quantities of hydrogen, to the bottom of the bed through a distributor. Entrainment of solid in the gas is limited by an appropriate dimensioning of the upper part of the reactor (freeboard, i.e. the space between the bed surface and the gas exit point), where the gas velocity is reduced, and, in some designs, by the interposition of cyclones in the exit gas line. The flow rate of the circulating gas is set so as to assure a velocity within an adequate range above the minimum fluidization velocity and below the xe2x80x9ctransport velocityxe2x80x9d. The heat of reaction is removed exclusively by cooling the circulating gas. The catalyst components may be fed in continuously into the polymerization vessel. The composition of the gas-phase controls the composition of the polymer. The reactor is operated at constant pressure, normally in the range 1-3 MPa. The reaction kinetics is controlled by the addition of inert gases.
A significant contribution to the reliability of the fluidized-bed reactor technology in the polymerization of a-olefins was made by the introduction of suitably pre-treated spheroidal catalyst of controlled dimensions and by the use of propane as diluent (see WO 92/21706). Since fluidized-bed reactors approximate very closely the ideal behaviour of a xe2x80x9ccontinuous stirred-tank reactorxe2x80x9d (CSTR), it is very difficult to obtain products which are a homogeneous mixture of different types of polymeric chains. In fact, the composition of the gaseous mixture that is in contact with the growing polymer particle is essentially the same for all the residence time of the particle in the reactor. As an example, one of the major limits of fluidized-bed processes is the difficulty of broadening the molecular weight distribution of the obtained polymers. It is generally known that, in the continuous polymerization of xcex1-olefins in a single stirred stage (which also involves steady composition of the monomers and of the chain transfer agent, normally hydrogen) with Ti-based catalysts of the Ziegler-Natta type, polyolefins having a relatively narrow molecular weight distribution are obtained. This characteristic is even more emphasised when metallocene catalysts are used. The breadth of the molecular weight distribution has an influence both on the Theological behaviour of the polymer (and hence the processability of the melt) and on the final mechanical properties of the product, and is a characteristic which is particularly important for the (co)polymers of ethylene. This problem has been addressed in WO 97/04015. According to this document, it is possible to broaden the molecular weight distribution of polymers without affecting their homogeneity by means of a gas-phase process performed in a loop reactor. The gas-phase polymerization according to WO 97/04015 is carried out in two interconnected polymerization zones to which one or more monomers are fed in the presence of a catalyst under reaction conditions and from which the polymer produced is discharged. The process is characterized in that the growing polymer particles flow through the first of said polymerization zones under fast fluidization conditions, leave said first polymerization zone and enter the second polymerization zone, through which they flow in a densified form under the action of gravity, leave the second polymerization zone arid are reintroduced into the first polymerization zone, thus establishing a circulation of polymer between the two polymerization zones.
According to the teachings of WO 97/04015, it is possible to broaden the molecular weight distribution of the polymers simply by properly balancing the gas-phase compositions and the residence times in the two polymerization zones of the gas-phase loop reactor. This is due to the fact that, while the polymer moves forward in the second polymerization zone flowing downward in a plug-flow mode, owing to the monomer consumption, it finds gas-phase compositions richer in molecular weight regulator. Consequently, the molecular weights of the forming polymer decrease along the axis of this polymerization zone. This effect is also enhanced by the temperature increase due to the polymerization reaction.
However, the process described in WO 97/04015 can provide only a limited control of the molecular weight distribution. In fact, even if hindered by the packed polymer, the diffusion of the gas within the polymerization zone in which the polymer particles flow in a densified form makes it difficult to establish substantial differences in the gas compositions at different heights of that zone. Moreover, it is not easy to achieve an effective balance of the residence times in the two different polymerization zones of the reactor.
Most importantly, WO 97/04015 gives no teaching on how to obtain homogeneous mixtures of polymeric chains having different compositions.
It would thus be desirable to improve the process of WO 97/04015 in order to be able to significantly broaden the molecular weight distribution of the obtained polymers and/or to render it suitable to the preparation of polymers endowed with broad composition distributions, while at the same time maintaining a high homogeneity level.
It has now been found that the above objectives, together with additional advantages, can be achieved by properly avoiding that the gas mixture present in the fast fluidized polymerization zone enter the densified solid flow polymerization zone.
Therefore, according to a first aspect, the present invention provides a process for the catalytic polymerization in the gas-phase carried out in at least two interconnected polymerization zones, the process comprising feeding one or more monomers to said polymerization zones in the presence of catalyst under reaction conditions and collecting the polymer product from said polymerization zones, in which process the growing polymer particles flow upward through one of said polymerization zones (riser) under fast fluidization conditions, leave said riser and enter another polymerization zone (downcomer) through which they flow downward under the action of gravity, leave said downcomer and are reintroduced into the riser, thus establishing a circulation of polymer between the riser and the downcomer, the process being further characterized in that:
(i) means are provided which are capable of totally or partially preventing the gas mixture present in the riser from entering the downcomer, and
(ii) a gas and/or liquid mixture having a composition different from the gas mixture present in the riser is introduced into the downcomer.
According to a particularly advantageous embodiment of the present invention, the introduction into the downcomer of the said gas and/or liquid mixture having a composition different from the gas mixture present in the riser is effective in preventing the latter mixture from entering the downcomer.
As it is known, the state of fast fluidization is obtained when the velocity of the fluidizing gas is higher than the transport velocity, and it is characterized in that the pressure gradient along the direction of transport is a monotone function of the quantity of injected solid, for equal flow rate and density of the fluidizing gas. Contrary to the present invention, in the fluidized-bed technology of the known state of the art, the fluidizing-gas velocity is maintained well below the transport velocity, in order to avoid phenomena of solids entrainment and particle carryover. The terms xe2x80x9ctransport velocityxe2x80x9d and xe2x80x9cfast fluidization statexe2x80x9d are well known in the art; for a definition thereof, see, for example, xe2x80x9cD. Geldart, Gas Fluidization Technology, page 155 et seq., J. Wiley and Sons Ltd., 1986xe2x80x9d.
Generally, in the downcomer the growing polymer particles flow downward in a more or less densified form. Thus, high values of density of the solid can be reached (density of the solid =kg of polymer per m3 of reactor occupied by polymer), which can approach the bulk density of the polymer. A positive gain in pressure can thus be obtained along the direction of flow, so that it becomes possible to reintroduce the polymer into the riser without the help of special mechanical means. In this way, a xe2x80x9cloopxe2x80x9d circulation is set up, which is defined by the balance of pressures between the two polymerization zones and by the head losses introduced into the system.
The gas mixtures involved in the process of the invention can contain entrained droplets of liquid composed of liquefied gas, as it is customary when operating in the so-called xe2x80x9ccondensing modexe2x80x9d. Generally, in the following description it is intended that a gas phase or a gas mixture can contain a part of entrained liquid.
According to a preferred embodiment of the invention, the introduction of the gas and/or liquid mixture of different composition into the downcomer is such to establish a net gas flow upward at the upper limit of the downcomer. The established flow of gas upward has the effect of preventing the gas mixture present in the riser from entering the downcomer.
Conveniently, the gas mixture coming from the riser is prevented from entering the downcomer by introducing the gas and/or liquid mixture of different composition through one or more introduction lines placed into the downcomer, preferably at a point close to the upper limit of the volume occupied by the densified solid. The flow ratio of the gas introduced and the downward velocity of the solid must be regulated so that a net flow of gas flowing upward is established at the upper limit of the zone into which the gas coming from the riser must not enter.
The gas and/or liquid mixture of different composition to be fed into the downcomer can optionally be fed in partially or totally liquefied form. The liquefied gas mixture can also be sprinkled over the upper surface of the bed of densified polymer particles; the evaporation of the liquid in the polymerization zone will provide the required gas flow.
The present invention is described with reference to the attached figures, which are given for illustrative purpose and not to limit the invention.